The frequency band that the broadcasters use for electronic news gathering (ENG) is transitioning from 17 MHz bandwidth channels to 12 MHz bandwidth channels. To maintain a substantially consistent level of quality, the current analog FM modulation scheme will be changed to a digital modulation scheme. The current workflow for establishing and maintaining a link between antennas typically requires aligning a simplex link using verbal feedback from the receive site back to the transmitter site. The feedback is usually provided by having an operator at a transmit site make a voice connection using 2-way radio or cell phone. The operator then performs initial antenna alignment in the analog FM domain using feedback from another operator at a receive antenna. Once antenna alignment is confirmed in the analog domain, a switch is then made to digital modulation. This process works adequately in a static environment. Once in the digital mode it is typically impossible to detect the presence of an adjacent channel transmitter that may cause interference until the picture starts to breakup or the BER deteriorates. By then, however, it is usually too late. When this occurs the user must switch back to analog mode to diagnose the problem.
In general, there are no tools currently available to adequately assist the operator in diagnosing link reliability issues in the digital mode. A solution that has been proposed and implemented is to add a spectrum analyzer to the 70 MHz intermediate frequency (IF) output of the receiver. This has proven to be a viable solution for isolating or identifying only a few problems related to on channel or co-channel interference. Such problems are incorrect modulation type of the transmitter or co-channel interference.
FIG. 1 is a diagram that illustrates a standard system configuration 100. As shown, a radio frequency (RF) signal 110 is received and amplified with an amplifier (LNA) 116. This signal is then down converted from the received RF frequency to a first IF frequency typically using a mixer 118 and synthesizer 122. The signal is then filtered with a wide band pass filter 126. The frequency of this first conversion is generally in the 800 MHz range. The specific frequency may vary based on the manufacturer's architecture. The signal is then further amplified at amplifier 130 and further down converted to a second IF frequency using mixer 134 and synthesizer 138. This is typically 140 or 70 MHz but not limited to these frequencies. The signal, now at a lower frequency, can be further filtered via filter 142 to remove adjacent channel signals located above and below the desired channel. After the filter 142, which is typically a narrow band-pass filter, the signal is usually further amplified using amplifier 146, i.e., automatic gain control (AGC), and applied to a demodulator 150 to recreate the original signal transmitted. Manufacturers of receivers will often provide an auxiliary IF output 156 for monitoring purposes. In this system architecture, a spectrum analyzer 160 can be connected external to the IF monitor point 156.
The system of FIG. 1 has generally been deployed and used to conduct link reliability analysis. The architecture of FIG. 1 has not, however, proven to be optimal for diagnosing the majority of link conditions that lead to link failure or reduce link reliability. A reason the aforementioned architecture is flawed is that many problems cannot be detected after filtering and after AGC amplification. Specifically, most interfering signals cannot be detected unless the transmitter is placed into standby mode, which results in the inability to see some types of adjacent or co-channel interference. Furthermore, when the transmitter is placed in standby mode, it is also very difficult to diagnose link problems because the AGC circuit in the receiver brings the noise floor or interference up to a level similar to the desired signal thereby disguising the real problem. This can cause operators or maintenance personnel to become confused and further complicate their job.
One place to monitor the link spectrum would be at the RF input. In this way, the full spectrum can be viewed relative the power levels of the channels of interest. But this architecture is relatively expensive as it duplicates the LNA and synthesizer circuitry. This method may also reduce the receiver's sensitivity because power intended for the receiver will be diverted to the spectrum analyzer.